The present invention relates to injection molding and, more particularly, to an improved method for adequately filling and compacting molding material in a mold chamber and at the same time avoiding troublesome and difficult problems such as drooling, stringing, and/or blockage of passageways leading to the mold chamber.
Generally, in an injection molding process, a mold chamber or cavity corresponding to the desired shape of an article to be molded is formed between two mold portions, one of which is movable to allow removal of the article upon completion of the process. After the mold is closed, the molding composition is injected under pressure and in a molten state through a nozzle into a runner network and in turn to individual cavities through sprues and on through connecting passageways or runners and through an opening or gate into the mold cavity or cavities. If a thermoplastic molding composition is used, the physical properties of such composition require that the mold cavity or cavities be cooled in order to solidify the composition before the mold can be opened to remove the molded article and to repeat the molding process. Accordingly, during the molding cycle, it is customary to maintain the cavity-containing portion of the mold cold so that the molding composition will be cooled quickly in the mold chamber or cavity.
During the cooling portion of the cycle, while the mold is open, and until the mold is closed to repeat the molding process, the molding composition in the several passageways leading to the mold chamber or cavity must remain in a molten state and not permitted to freeze since this would thereby prevent further operation of the molding apparatus.
In addition to the aforementioned conflicting requirements, there are additional difficult problems which arise from other conflicting requirements relating to the performance of the gate or orifice that connects the aforesaid passageways with the mold chamber or cavity. If the area of the gate is too restricted, it is difficult to adequately fill and pack the mold chamber or cavity and/or the filling and packing process is excessively time-consuming. On the other hand, if the area of the gate is too large, solidification of the molding composition in the vicinity of the gate is inadequate or incomplete with the result that undesirable drooling and/or stringing of the molding composition occurs when the mold is opened.
Certain prior art improvements such as thermally insulated runners, heated gate probes, and restricted orifice type of gate design allow the use of restricted gates without encountering serious freezing problems, and thereby provide at least a partial solution to some of the above-noted difficult problems. Also, valve gating has met with some degree of success in overcoming drooling and/or stringing problems but valve gating presents an extremely complex design problem and is very expensive to incorporate into a multi-cavity mold.
Accordingly, a principal object of the present invention is to provide improved means and method for obviating the above-noted difficult problems.